Simplify the following expression: $p = \dfrac{1}{9r} - \dfrac{3}{4r}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9r$ and $4r$ $\lcm(9r, 4r) = 36r$ $ p = \dfrac{4}{4} \cdot \dfrac{1}{9r} - \dfrac{9}{9} \cdot \dfrac{3}{4r} $ $p = \dfrac{4}{36r} - \dfrac{27}{36r}$ $p = \dfrac{4 -27}{36r}$ $p = \dfrac{-23}{36r}$